1. Field of the Invention
The present invention relates to a bipolar transistor including an insulated gate (MOS structure), and more particularly to a structure of an insulated gate bipolar transistor (which will hereinafter be abbreviated as an “IGBT”, and may be also referred to as a “reverse conducting IGBT” in general) including a built-in freewheeling diode (which will hereinafter be also abbreviated as a “FWD”), and a technique for manufacturing the same. The IGBT according to the present invention can be used as a switching device with a built-in FWD in an inverter circuit for driving a load such as a motor, for industrial purposes.
2. Description of the Background Art
In power electronics for driving a motor or the like, under a condition that a rated voltage is 300V or higher, an IGBT is usually used as a switching device because of its characteristics. In using an IGBT as a switching device, a freewheeling diode (FWD) which is connected in parallel with the switching device is also used.
In a typical IGBT developed in the conventional art, an N+-type buffer layer is formed on a P+-type collector layer, an N−-type layer is formed on the N+-buffer layer. Also, a P-type base region is selectively formed on a surface of the N−-type layer as a result of diffusion of P-type impurities. Further, a source region is formed on a surface of the P-type base region as a result of selective diffusion of a high concentration of N-type impurities. The P-type base region and the source region are formed using a polysilicon gate as a mask. Because of inclusion of an area including the P-type region and the source region, in which double diffusion is caused, an overall structure is referred to as a “Double Diffused MOS”, or abbreviated as a “DMOS”. On the surface of the N−-type layer on which the P-type base region and the source region are formed, a gate oxide film is formed. Moreover, a gate electrode of polysilicon is formed on an upper portion of the gate oxide film, and thus a portion of the P-type base region which is located under the gate electrode will serve as a channel region. Furthermore, an emitter electrode is formed so as to extend over a portion of a surface of the source region of N+-type and a central portion of a surface of the P-type base region, and a collector electrode is formed on a back surface of an N+-type substrate.
As an alternative to the above-mentioned IGBT, a trench IGBT in which a gate electrode of a MOS is formed within a trench provided inside of silicon has been developed in the conventional art.
It is noted that an inverter circuit functions to change a dc voltage into an ac voltage. A typical inverter circuit includes the above-mentioned IGBT serving as a switching device and a freewheeling diode (FWD). The inverter circuit controls a two-phase or three-phase ac motor by employing a combination of four or six circuit elements each formed of the IGBT and the freewheeling diode connected in parallel with the IGBT. More specifically, the inverter circuit includes a dc terminal connected to a dc power supply, and causes each of the IGBTs to perform a switching operation, to thereby change a dc voltage to an ac voltage with a predetermined frequency, thereby supplying the ac voltage to a motor serving as a load.
The following prior art documents may be referred to: Japanese Patent Application Laid-Open No. 6-196705 (FIG. 1) (which will hereinafter be referred to as “JP 6-196705”); Japanese Patent Application Laid-Open No. 7-153942 (FIG. 1) (which will hereinafter be referred to as “JP 7-153942”); Japanese Patent Application Laid-Open No. 6-53511 (FIG. 1) (which will hereinafter be referred to as “JP 6-53511”); Japanese Patent Application Laid-Open No. 2-126682 (which will hereinafter be referred to as “JP 2-126682”); and Japanese Patent Application Laid-Open No. 8-116056.
The freewheeling diode is required in the conventional inverter circuit as described above because the motor serving as a load is inductive. Regarding this issue, details will be provided as follows.
The inductive load has a property of storing energy in a magnetic field generated by a current. Accordingly, change in a current means change in stored energy. In the present specification, a storage ability of an inductive load will be represented by “L”. Upon interruption of a current flowing through the load, energy stored in L of the load is released by a matter which is attempting to interrupt the current, so that the energy will function to prevent change in the current. Instant release of the energy stored in the L of the motor leads to generation of an electric power which is high enough to degrade characteristics of the IGBT. Thus, when the IGBT performs a switching operation to suddenly interrupt the current flowing through the motor, the characteristics of the IGBT is significantly degraded because of the released energy.
In view of this, the freewheeling diode is provided, to cause the current flowing through the motor during an off state of the IGBT to freewheel through a bypass path, in order to prevent the current flowing through the motor from being changed under influence of the switching operation. For this reason, in the typical inverter circuit in the conventional art, the dc power supply and the motor are connected to each other. Thus, when the IGBT is turned off to stop applying a voltage to the motor, the current flowing through the motor flows through the freewheeling diode to thereby reverse the course as a direct current because of the energy stored in the L of the motor. As a result, the motor is placed in a state equivalent to a state where a reverse dc voltage is applied to the motor. Changing a ratio between a turn-on time period and a turn-off time period of the IGBT leads to change in a ratio between a time period during which a dc voltage is applied and a time period during which a reverse current is flowing. Accordingly, a voltage applied to the motor can be controlled to be uniform.
As such, by changing the ratio so as to become sinusoidal, it is possible to allow the IGBT to perform a switching operation to thereby supply an ac voltage from the dc power supply while preventing the current flowing through the motor from being suddenly interrupted because of the switching operation of the IGBT.
Because of the foregoing operating manner of the inverter circuit, there is a need of providing the freewheeling diode inverse-series connected to a given IGBT, or providing the freewheeling diode anti-parallel connected to another IGBT which is paired with the given IGBT, as described above.
In this regard, a conventional power MOSFET which is also used as a switching device does not require additionally connecting a freewheeling diode when the power MOSFET is used as a switching device of an inverter circuit, because the power MOSFET includes a built-in anti-parallel connected diode. However, a density of a conductible current of the power MOSFET is relatively low, and thus the power MOSFET is unsuitable for high current applications.
Hence, there is no choice but to employ an IGBT as a switching device of an inverter circuit for driving a motor or the like. However, the IGBT has a structure formed by changing a portion out of an N+-type layer which is located on a side of a drain electrode, to a P+-type layer in a substrate of a power MOSFET, and thus a diode is formed between a P+-type collector layer in a back surface and an N+-type buffer layer thereon. A breakdown voltage (a forward drop voltage Vf) of the diode is in a range approximately from 20V to 50V. Such voltage is too high as a breakdown voltage of a freewheeling diode. Because of presence of a barrier having such a high breakdown voltage, characteristics of the IGBT may be significantly degraded upon generation of heat due to a voltage applied during freewheeling. For this reason, while an IGBT is advantageous to a power MOSFET in view of a density of conductible current, the structure of the IGBT could not allow inclusion of a built-in diode, unlike a MOSFET, and therefore there is still a need of additionally connecting a freewheeling diode manufactured independently of the IGBT in the conventional inverter circuit employing the IGBT as a switching device.
As a consequence of the foregoing, to incorporate a diode into an IGBT in the same manner as a diode is incorporated in a power MOSFET which was developed earlier than the IGBT has been a concern in technologies. To this end, various approaches have ever been proposed.
For example, in a structure proposed in JP 7-153942, incorporation of a diode into an IGBT is achieved by forming an N+-type layer which extends through a P+-type collector layer in a back surface of the IGBT. Also, in a structure proposed in JP 6-53511, incorporation of a diode into an IGBT is achieved by locally providing a portion of a P+-type collector layer in a back surface of the IGBT such that a portion of the P+-type layer extends into an N+-type layer. However, it should be noted that both JP 7-153942 and JP 6-53511 mention the above structures as wishful thinking, and the above structures have not yet be put into practical use, for the following reasons. Most of IGBTs commercially available have a reverse breakdown voltage of 600V or 1200V and an N−-type layer must have a thickness in a range from 50 μm to 150 μm in order to maintain the breakdown voltage. On the other hand, a wafer must have a thickness in a range from 250 μm to 600 μm in order to undergo a wafer process (W/P). Hence, the P+-type collector layer in the back surface will have a thickness of 100 μm or larger. Accordingly, it is difficult in practice to form an N-type polycrystalline region configured so as to extend through the P+-type collector layer in the structure of JP 7-153942. Turning to the structure of JP 6-53511, the thickness of the N−-type layer is too thick to implement a structure allowing flow of a current during W/P, which results in failure to benefit particular effects from the characteristics of the IGBT.
Alternatively, JP 2-126682 proposes connecting a portion of an N−-type layer to a collector electrode in order to improve the characteristics of an IGBT. However, in JP 2-126682, since a diode does not have satisfactory characteristics and thus is unsuitable for use, a structure which makes the diode inoperable is disclosed.
Further, JP 6-196705 discloses a similar structure. Specifically, JP 6-196705 discloses a structure in which a P−-type layer is formed in a P-type layer located on a side of a top surface, in order to improve recovery characteristics of a built-in diode. JP 6-196705 further teaches that an N−-type layer has a thickness of 50 μm and a P+-type collector layer has a thickness of 20 μm. Moreover, according to a manufacturing method described in JP6-196705, an N−-type substrate is prepared first, the P+-type collector layer and an N+-type cathode layer in the back surface are formed, and then a MOSFET in the top surface is formed. The method of JP6-196705 requires carrying out all steps in W/P with a thickness of a wafer being maintained at approximately 80 μm. Thus, the method is disadvantageous in that it is extremely difficult to handle the wafer during the W/P.